A Budding Yeast Model System to Define Biological Pathways Altered by Pathogenic Missense Mutations in Histone Genes Identifies a Link between Histone H3K36 and the TOS4 Gene

Histones are critical for both packaging DNA into chromatin within the cell nucleus and for regulating all aspects of gene expression. DNA is packaged into nucleosomes, which consist of a hetero-octamer of histones H2A, H2B, H3, and H4. Somatic missense mutations in histone genes turn these essential proteins into oncohistones, which can drive oncogenesis. These mutations, termed oncohistone mutations, have been linked to multiple types of cancer, including pediatric gliomas and head and neck cancers. In humans, a histone H3 lysine 36 to methionine missense mutation (H3K36M) causes chondroblastomas. Our lab has employed a budding yeast model system to explore how missense mutations such as H3K36M alter histone function. These studies, as well as those from previous groups, reveal that changes at H3K36 confer sensitivity to growth on plates containing caffeine, suggesting that H3K36 mutants show changes in stress response pathways. To further define these pathways, the laboratory employed a high copy suppressor screen to identify genes that when overexpressed could suppress the caffeine-sensitive growth phenotype of H3K3M cells. This screen identified several genes linked to epigenetic regulation, including TOS4, which encodes a forkhead-associated (FHA) domain protein that interacts with Rpd3L and Set3 histone deacetylase complexes. To begin to define the function of Tos4 required for this suppression, two amino acid substitutions within the Tos4 FHA domain that disrupt interaction with histone deacetylase complexes (R122A, N161A) were generated. This Tos4 variant still suppresses the caffeine-sensitive growth of H3K36M cells, suggesting these interactions are not required for suppression. My studies focus on defining the function of Tos4 required for this suppression. Results from such studies may provide insight for novel therapeutic targets in humans diagnosed with these distinct oncohistone-driven cancer types.